A three-terminal device whose output current, voltage, and/or power are controlled by its
input current (Bipolar Junction Transistor, BJT) or voltage (Field-Effect Transistor, FET).
Function of transistor
1. Amplification (BJT)
a circuit used to increase the strength of an ac signal.
2. Switching (FET)
switch “on” and “off” at a rate of several billions of times per second (~ GHz)
Type of transistor
Bipolar junction transistor (BJT)
Field effect transistor (FET)
Transistors
This picture shows the workbench of John Bardeen and Walter Brattain
at Bell Laboratories. They were supposed to be doing fundamental
research about crystal surfaces. The experimental results hadn't been
very good, though, and there's a rumor that their boss, William
Shockley, came near to cancel the project.
But in 1947, they switched to pure materials. And it dawned on them
that they could build the circuit in the picture. It was an amplifier!
The three shared a Nobel Prize. Bardeen and Brattain continued in
research (and Bardeen later won another Nobel). Shockley quit to start
a semiconductor company in Palo Alto. It folded, but its staff went on to
invent the integrated circuit (the "chip") and to found Intel Corporation.
Bipolar junction transistors (BJTs)
The BJT is a transistor with three regions and two pn junctions. The regions are named
as the emitter, base, and collector and each is connected to a lead.
There are two types of BJTs – npn and pnp.
1. Arrow on the emitter terminal.
2. Arrow from p-type to the n-type material.
( direction of hole current flow)
3. Arrow along the direction of emitter current.
npn transistor
• Under normal circumstances, IE has the greatest value of
the three, a little bit larger than IC .
• IB normally has the smallest value than the others
• IE and IC are controlled by IB
• forward current gain, β ~ a factor by which current
increases from the base of a transistor to its collector.
where ~ 100
11
B C E
E B C
C B
C E
I I I
I I I
I I
I I
Terminology
Operation of npn Transistor
B-E junction B-C junction Mode Voltages
Forward Reverse Forward-active (활성) VE < VB < VC
Forward Forward Saturation (포화) VE < VB > VC
Reverse Reverse Cut-off (차단) VE > VB < VC
Reverse Forward Reverse-active VE > VB > VC
There is an avalanche breakdown mode for very high VCE
Zero Biasing at room temperature with no potentials applied
The depletion layer extend farther into the base region than either of the other two because the base region has a lower doping level
++
+
Cut-off (Reverse, Reverse)
Both pn junction in a transistor are reverse biased.
IC is nearly zero even with the most high VCE
B-E junction B-C junction Mode Voltages
Reverse Reverse Cut-off (차단) VE > VB < VC
Saturation (Forward, Forward)
IC reaches its maximum value of CCC
C E
VI
R R
n-type collector is lightly doped than the emitter region,
for the built-in potential to be +0.4 V between the base
and the collector.
No more increase of IC even with more larger IB
Active (Forward, Reverse)
When VBE is sufficient to overcome the barrier potential of the junction, current is generated in the emitter and base region (IBE)
The BJT operates between saturation (maximum IC) and cutoff (minimum IC) regions
Transistor operating regions
6.3.1 Transistor Currents
current controlled (current driving) device
: a small change in IB results in larger change in the other terminal current
EXAMPLE 6.1
Determine the value of collector current for the values of base current shown in the figure.
6.3.2 The Relationship Among IE, IC, and IB
From Kirchhoff’s current law,
IE = IB + IC for npn transistor
Since IB is normally much less than IC,
IE ≈ IC for npn transistor
6.3.3 DC Beta
The ratio of dc collector current to dc base current
The most common transistor circuits have an input signal applied to the base and an output signal taken from the collector
EXAMPLE 6.3
Determine the values of IC and IE for the circuit shown in the figure.
EXAMPLE 6.4
Determine the values of IB and IE for the circuit shown in Figure 6.14.
6.3.4 DC Alpha
The ratio of dc collector current to dc emitter current.
Also referred to as collector current efficiency.
6.3.5 The Relationship Between Alpha and Beta
EXAMPLE 6.5
Determine the value of alpha for the transistor shown in Figure 6.16. Then, determine the value of IC using both the alpha and the beta ratings of the transistor.
6.3.6 Maximum Current Ratings
Most transistor spec sheets list maximum collector current ratings (IC(max))
The maximum allowable base current for a given IC(max) :
EXAMPLE 6.6
The transistor shown in Figure 6.17 has the following ratings: IC(max) = 500 mA and βmax = 300. Determine the maximum allowable value of IB for the device.
6.4 Transistor Characteristic Curves
6.4.1 Collector Curves
Kirchhoff’s law
Assuming VBE = 0.75 V
Saturation Mode !!
6.4.1 Collector Curves
400 W
IC is determined by IB and .
Active Mode !!
Changing the value of RC has not affected the value of IC. But, VCE changed from 0.3 to 6 V
IC is not controlled by VCE when a transistor is operated in the active region.
A composite of collector characteristic curves for a BJT
6.4.2 Base Curves
6.6 Transistor Testing